new Promise((resolve) => {
    setTimeout(() => {
        var a = 'hello'
        resolve(a)
    }, 10)
}).then(res => {
    return new Promise((resolve) => {
        setTimeout(() => {
            var b = ' lagou'
            resolve(res + b)
        }, 10)
    })
}).then(res => {
    return new Promise((resolve) => {
        setTimeout(() => {
            var c = ' I ❤  U'
            resolve(res + c)
        }, 10)
    })
}).then(console.log)